Caledonian Railway 721 Class

Caledonian Railway 721 Class
"Dunalastair"
For variations see Table 1 below

Type and origin Power type Steam Designer John F. McIntosh Builder St. Rollox railway works Build date 1896 Total produced 15

Specifications Configuration 4-4-0 Gauge 4 ft 8 1⁄2 in (1,435 mm) Driver dia. 6 ft 6 in (1.98 m) Loco weight 47 long tons (48 t) Boiler pressure 160 psi (1,100 kPa) Cylinders Two, inside Cylinder size 18.25 in × 26 in (464 mm × 660 mm) Valve gear Stephenson Valve type Slide valves

Performance figures Tractive effort 15,100 lbf (67 kN)

Career Operators CR • LMS • British Railways Class CR: 721 Power class LMS: 2P Withdrawn 1930–35 Disposition All scrapped

Type and origin
Power type	Steam
Designer	John F. McIntosh
Builder	St. Rollox railway works
Build date	1896
Total produced	15

Specifications
Configuration	4-4-0
Gauge	4 ft 8 1⁄2 in (1,435 mm)
Driver dia.	6 ft 6 in (1.98 m)
Loco weight	47 long tons (48 t)
Boiler pressure	160 psi (1,100 kPa)
Cylinders	Two, inside
Cylinder size	18.25 in × 26 in (464 mm × 660 mm)
Valve gear	Stephenson
Valve type	Slide valves

Performance figures
Tractive effort	15,100 lbf (67 kN)

Career
Operators	CR • LMS • British Railways
Class	CR: 721
Power class	LMS: 2P
Withdrawn	1930–35
Disposition	All scrapped

The Caledonian Railway 721 Class (known as the "Dunalastair" class) was a class of 4-4-0 steam locomotives designed by John F. McIntosh for the Caledonian Railway (CR) and introduced in 1896. All survived to be absorbed by the London, Midland and Scottish Railway (LMS) in 1923 and a few survived into British Railways (BR) ownership in 1948.

The "Dunalastair" class marked a new era of development in late-Victorian British steam locomotive design. The average weight of passenger trains had greatly increased in the 1880s and 1890s due to the demand for more comfortable, better-appointed and safer carriages. Combined with continually rising passenger volumes and the competition between railway companies to offer faster services and locomotive engineers were faced with producing engines that could operate longer, heavier trains at faster speeds. The existing pattern of inside-cylinder 4-4-0 express engine was reaching the limits of its development and many railways were resorting to the inefficient practise of double heading to maintain schedules.

McIntosh provided the solution with the original "Dunalastair" of 1896. In broad design this was identical to the conventional 4-4-0 engine drawn up by his predecessor Dugald Drummond but it carried a boiler significantly larger than was usual for the time - almost to the full limits that the Caledonian's loading gauge would allow, operating at a relatively high pressure of 160psi (1100kPa). The boiler also contained more fire tubes of a greater diameter than its predecessors, greatly increasing its steam generating capacity and overall steam volume. While the top speed of the 721 Class and its developments was largely the same as other express 4-4-0s the more productive and capacious boiler meant the type offered a hugely improved ability to maintain high speeds with heavy loads on steep gradients, allowing the "Dunalastairs" to set high average speeds over the Caledonian's arduous main line over . So confident was McIntosh that his new engine would do away with the need for double heading that the "Dunalastairs" were originally built without brake hoses on their front buffer beam, meaning that a second engine could not be coupled to them.

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